BRISK Feature Detector detects no keypoints

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The Brisk detector shown below gives me no keypoints. Can somebody please suggest a problem. I will try to explain what I am doing below with some of the codes.

#include "opencv2/features2d/features2d.hpp"

using namespace cv;    
using namespace std;

Ptr<FeatureDetector> detector;
detector =  FeatureDetector::create("BRISK");

// the filename is given some path

Mat img = imread(filename, 0);
CV_Assert( !img.empty() );

vector<KeyPoint> kp;

detector->detect(img, kp);

So, when I debug this and check the keypoint (kp) it says <0 items>

with similar code other detector like ORB, SIFT or SURF work as intended!

Can somebody please suggest a solution.

I am using opencv 2.4.9 with Qt creator 2.5.2

Thanks